Add option to disable offline error

I run into the offline [error popup](https://github.com/atom-community/sync-settings/blob/450491cf231ee4e4cc6864ef97ed81e161efc54a/lib/location/gist.js#L219) _constantly_ as I often have to use my laptop in an environment where I don't have an internet connection. It's really frustrating to have to dismiss it every time I open Atom.

Honestly, it seems like this error should be disabled by default. Syncing settings is the kind of thing where I expect it to only bother me when it really needs to. No-one needs it to complain that you're offline on startup. Maybe when you manually trigger the sync from the menu I could see it making sense to do so.

Please silence the error for auto-sync on startup, or at least add an option to do so.

Start at lib/location/gist.js around line 219, then trace how startup auto-sync and menu-triggered sync reach the offline error. Decide whether the implementation follows the requested default silence or adds an option, and verify that offline startup no longer shows the popup while any intended manual-sync behavior remains clear.

Written by the indexing model from the issue text.